ALIASING

ALIASING is a phenomenon that can occur during any form of SAMPLING that in digital image processing can lead to picture ARTEFACTS.

The diagram shows the sampling of a high frequency signal at a high sampling rate and a low sampling rate. There are enough sample points at the high sampling rate give to give an accurate representation of the high frequency signal. However at the lower sampling rate, there is not only no representation of the original signal, there is a clear but erroneous representation of a much lower frequency signal, the frequency of which is a function of the periodicities of the sampling rate and the original signal.

Sampling in digital images can be both spatial or temporal. Spatial sampling is defined by the resolution of the picture – an HD image has 1920 horizontal samples. Temporal sampling depends on the frame rate – in cine film a full frame is recorded every 1/24 second.

Aliasing in spatial sampling occurs particularly when trying to resolve patterns and can result in the distinctive moire-pattern seen on the siemens star to the right. The curved patterns are caused by the computer screen, with its limited number of pixels arranged in a grid arranged horizontally and vertically, failing to fully resolve the fine detail especially the diagonal lines at the centre of the star.

Temporal aliasing can be seen in the familiar effect of car or bicycle wheels appearing to rotate in the wrong direction when seen on film. This is particularly noticeable when a wheel is filmed accelerating from a standstill. Up to a certain speed we see the wheel rotating faster and faster. At a certain speed the film camera’s frame rate cannot resolve the incremental turning of the wheel and an illusion occurs where the eye interprets a spoke which has not quite completed a full revolution between frames as actually moving backwards. This effect is greater at short exposure times – MOTION BLUR can hide aliasing ARTEFACTS.

Another example of aliasing: if you’ve ever been in a club with a crowd bouncing up and down to upbeat music when the strobe comes on, you will have noticed that people look like they’re dancing in slow motion even though you know they are moving rapidly, or “having it”. This is an example of under sampling causing aliasing – the frequency of the strobe is not high enough to resolve the crowd’s rapid, repetitive movements.

If you scroll this page up and down and look at the siemens star while scrolling, the moire-patterning increases and changes shape. This is due to a combination of spatial aliasing and temporal aliasing as the screen refreshes.